In order to purify combustion gas (exhaust gas) discharged from an internal combustion engine of a motor vehicle, three-way catalysts are widely used. A three-way catalyst reduces or oxidizes CO (carbon monoxide), HC (hydrocarbon) and NOx (oxide of nitrogen) contained in exhaust gas into water, carbon dioxide and nitrogen to purity the exhaust gas. Such a three-way catalyst includes a support formed of a metal oxide (metal oxide support), and a noble metal material such as platinum (Pt), rhodium (Rh), palladium (Pd) or the like supported on the support.
As the metal oxide support, it is conventionally common to use alumina (Al2O3) in order to provide a relatively large specific surface area. However, it has recently been proposed to use a metal oxide other than alumina such as ceria (CeO2), zirconia (ZrO2), titania (TiO2) or the like in stead of, or in combination with, alumina in order to utilize chemical characteristics of the metal oxide support to further improve the purification performance.
Studies are also made on preferable combinations (chemistry) of a metal oxide support and a noble metal material. It has been reported that rhodium, when used in combination with a metal oxide containing zirconia as a main component (zirconia or zirconia-based complex oxide), provides superb purification performance.
When rhodium is supported by alumina, which is conventionally used commonly, rhodium is dissolved into alumina as time passes, and this decreases the catalyst activity. By contrast, when rhodium is supported by a metal oxide containing zirconia as a main component, rhodium is not dissolved. Therefore, the high catalyst activity which rhodium originally has can be utilized.
However, it is difficult to cause a metal oxide containing zirconia as a main component to adsorb and thus support rhodium. When a metal oxide containing zirconia as a main component is merely mixed in a commercially available aqueous solution of rhodium, rhodium is not adsorbed to the metal oxide support almost at all.
Patent Documents 1 and 2 each disclose a technique for causing a metal oxide containing zirconia as a main component to support rhodium.
According to the technique disclosed in Patent Document 1, zirconia is added to an aqueous solution of rhodium nitrate and the resultant solution is evaporated to dryness, so as to cause zirconia to support rhodium. According to the technique disclosed in Patent Document 2, a metal oxide containing zirconia as a main component is immersed in a colloidal solution containing rhodium, so as to cause the metal oxide support to support colloidal rhodium.